A microgrid is a semiautonomous or fully autonomous grouping of distributed energy resources (distributed generation, energy storage) and controllable loads within a local area. The loads can be one utility “customer,” a grouping of several sites, or dispersed sites that operate in a coordinated fashion also known as aggregated loads. The distributed electric generators can include reciprocating engine generators, microturbines, fuel cells, photovoltaic/solar, gas turbines and other small-scale renewable generators. All controllable distributed energy resources and loads are interconnected in a manner that enables devices to perform certain microgrid control functions. For example, the energy balance of the system must be maintained by dispatch and non-critical loads might be curtailed or shed during times of energy shortfall or high operating costs. While capable of operating independently of the grid (in islanded mode), the microgrid usually functions interconnected (in grid-connected mode) with a substation or main grid both of which are referred to herein as a grid, purchasing energy from the grid or system operator and potentially selling back energy and ancillary services at different times. Microgrids are typically designed based on the total system energy requirements of the microgrid also known as net-metering. Heterogeneous levels of power quality and reliability are typically provisioned to end-uses. A microgrid is typically presented to the grid as a single controllable entity.
Conventional distribution grids were designed as passive networks in which power flows from a transmission grid to end customers. However, due to large penetration of renewables and active loads such as electrical vehicles, residential PV and storage the distribution grid is becoming more dynamic. The dynamic nature of a distribution grid poses several challenges in terms of power flow and control. In addition, extreme weather conditions have exposed vulnerabilities of traditional distribution grids. Operating distribution circuits such as microgrids provide a unified elegant solution to these problems. One advantage of a microgrid is that it can operate in both grid-connected mode and islanded mode. However, there are some challenges in both of these operational modes. For example, an islanded microgrid requires re-synchronization before it can be re-connected to the grid (i.e. a substation or main grid). Re-synchronization involves ensuring bus voltages, frequency and bus angle ideally match or are at least within some acceptable tolerance. Conventionally, a single large grid-forming diesel generator or the largest inverter in a microgrid with grid forming capability performs the re-synchronization. Accordingly, only the diesel generator or PV control system parameters can be adjusted to achieve synchronization. As such, there is a need for a more flexible and robust microgrid re-synchronization approach.